The present invention relates to a unique word differential detection method and a demodulator using the unique word differential detection which detect a known unique word (UW) and a carrier frequency offset of a quasi-synchronized detection signal by using a differential detection technique of the quasi-synchronized detection signal which quasi-synchronously detects an orthogonal modulation signal in which the known unique word is inserted in a data signal.
Such a unique word differential detection system has a feature that, when the orthogonal modulation signal is quasi-synchronously detected by using a local oscillator signal independent from the orthogonal modulation signal, the unique word can be detected even if there is a frequency difference between the carrier of the orthogonal modulation signal and the local oscillator signal, that is, even if there is a carrier frequency offset. In addition, it also has a feature that the unique word can be detected at a high speed. A unique word detector disclosed in Japanese Patent Application Laid-Open No. 5-167630 is one of such unique word differential detection systems. Now, a conventional unique word differential detection system is described with reference to FIGS. 1, 2 and 3.
FIG. 1 is a format of data signal according to the present invention. FIG. 2 is a functional block diagram showing a unique word differential detection system according to the related art. The unique word differential detection circuit shown in the figure is a basic circuit for the unique word differential detection system according to the present invention. In addition, FIG. 3 is a diagram for illustrating the operation of the unique word differential detection system of FIG. 2.
Referring to FIG. 1, a base-band data signal according to the present invention constitutes one frame with a unique word (hereinafter sometimes abbreviated to UW) having a known L symbol train and data of a plurality of symbols. The UW is used for a frame signal. The data signal typically has a UW of 30-50 symbols, data of 200-400 symbols, and one frame interval Tf of 40-200 milliseconds (mS). The UW is positioned at the top of each frame. A symbol repetition frequency is represented by Fs (symbol/S), and one symbol interval is represented by Ts=1/Fs (S).
Referring to FIG. 2, a UW differential detector circuit 100 is supplied with a received quasi-synchronized detection signal S100=R (t) and a local unique word Suw=U (t) obtained by generating a known UW. Then, it outputs a mutually correlated signal S107=D (t) which is a signal which differentially detects the signal S100 mutually correlated with a signal which differentially detects the unique word Suw. The quasi-synchronized detection signal S100 is a signal which quasi-synchronously detects the orthogonal modulation signal with the known UW inserted in the data signal. The quasi-synchronized detection means synchronously detects the orthogonal modulation signal with a local oscillation signal independent from the orthogonal modulation signal but with a frequency close to that of the orthogonal modulation signal, and demodulates the orthogonal modulation signal into the data signal at base band. Generally, there is a frequency difference F.sub.0 (Hz/S) or a carrier frequency offset (hereinafter called the "frequency offset") F.sub.0 between the carrier of the orthogonal modulation signal and the local oscillation signal.
The quasi-synchronized detection signal S100 consists of two trains of in-phase (I) signals and quadrature (Q) signals. In the following, by considering that the quasi-synchronized detection signal S100 is a complex signal of I and Q signals, the in-phase component of a signal R (t) may be referred to as ReR (t) meaning a real signal component, and the quadrature signal component may be referred to as ImR (t) meaning an imaginary signal component. The signal R (t) is a signal with an inverted code "1" or "-1". ReR (t) and ImR (t) of the quasi-synchronized detection signal S100 are inserted with the same UW in the same timing, respectively. The quasi-synchronized detection signal R (t) is expressed by Formula (1) if there is the frequency offset F0. EQU R (t)=S (t).multidot.e.sup.j2 .pi.(F0.multidot.t+.theta.a) (1)
where S (t) is data to be transmitted, and assumed to be QPSK modulated. EQU S (t)=e.sup.2 .pi.(k(t)/4) (2)
where k (t) is the number of modulation phase of the orthogonal modulation signal at time t, and .theta.a is any phase. When it is assumed that an L symbol interval of the UW is .tau. (0&lt;.tau.&lt;L.multidot.Ts=L/Fs), and n is a frame number, a unique word U (t) is expressed by Formula (3) in a unique word interval .tau.. EQU U (.tau.)=S (n.multidot.Tf+.tau.) (3)
The UW differential detector circuit 100 delays the quasi-synchronized detection signal S100=R (t) with the delay circuit 101a by N symbols (=N.multidot.Ts seconds) (N is any positive number), and the N symbol delayed signal S101 is complex conjugated by a complex conjugate circuit 102a. That is, the complex conjugate circuit 102a inverts the code of ImR (t) of the N symbol delayed R (t), and N symbol delays and complex conjugates the quasi-synchronized detection signal R (t) to generate a complex conjugate signal S102. A multiplier 103a multiplies the quasi-synchronized detection signal S100 with the complex conjugate signal S102, that is, N symbol complex conjugates, delays and detects the quasi-synchronized detection signal S100 to generate a data signal differential detection signal S103. The data signal differential detection signal S103 in a .tau. interval is expressed by Formula (4), where R* is the complex conjugate of the quasi-synchronized detection signal R (t). EQU S103=R (n.multidot.Tf+.tau.).multidot.R* (n.multidot.Tf+.tau.-N.multidot.Ts)(4)
On the other hand, a unique word Suw=U (t) of the base band repeating the same signal train as the unique word in the quasi-synchronized detection signal S100 is input into the delay circuit 101b and the complex conjugate circuit 102b. The unique word Suw may insert a signal in the same format as the quasi-synchronized detection signal S100 therebetween. The unique word U (t) also consists of two trains, a real signal component ReU (t) and an imaginary signal component ImR (t). The delay circuit 101b generates an N symbol delayed signal S104 which is the unique word Suw=U (t) delayed by N symbols. The complex conjugate circuit 102b complex conjugates the unique word U (t), that is, inverts the code of ImU (t) to generate a complex conjugate signal S105. A multiplier 103b multiplies the N symbol delay signal S104 with the complex conjugate signal S105, N symbol complex conjugates, delays and detects the unique word Suw to generate a UW differential detection signal S106. The UW differential detection signal S106 is expressed by Formula (5), where U* is complex conjugate of the unique word U (t). The resultant UW differential detection signal S106 is sent to a correlator 104, and stored as a reference for correlation detection. EQU S106 (.tau.)=U (.tau.-N.multidot.Ts).multidot.U* (.tau.) (5)
The correlator 104 cross correlates the data signal differential detection signal S103 being sequentially sent and the stored UW differential detection signal S106 over L symbols, or over the entire symbol length of the UW. If N=1, the correlator 104 generates a cross correlation signal S107=D (t) as understood by Formula (6). ##EQU1##
If t=0 where timing of the unique word in the quasi-synchronized detection signal R (t) matches timing of the unique word U (t), Formula (6) can be expressed by Formula (7) EQU S107 (t=0)=(L/Fs).multidot.e.sup.j2 .pi.F.sbsp.0.sup.Ts (7)
In Formula (7), the cross correlation signal S107=D (t) has an amplitude of (L/Fs) and a value of phase angle .theta. of (2 .pi.F0.multidot.Ts). That is, a phase term .theta. is proportional to a product of the frequency offset F.sub.0 and the delay time (N.times.Ts). Alternatively, the frequency offset F.sub.0 is expressed as F.sub.0 =.theta./(2 .pi.Ts)=.theta..multidot.Fs/2 .pi..
The correlator 104 divides the cross correlation signal S107=signal D (t) into a real component Re and an imaginary component Im, and outputs them. That is, the cross correlation signal S107 has a relationship of (L/Fs)=(Re.sup.2 +Im.sup.2).sup.1/2 for amplitude, and a relationship of Re=(L/Fs).multidot.cos .theta., Im=(L/Fs).multidot.sin .theta. for the phase term .theta..
A UW detector 105 squares the Re and Im of the cross correlation signal S107, respectively, to generate a power value (L/Fs).sup.2. If timing of the unique word in the quasi-synchronized detection signal R (t) matches timing of the unique word U (t), the power value (L/Fs).sup.2 produces a peak at the position of the last symbol of the unique word Suw. The UW detector 105 compares the power value (L/Fs).sup.2 and a predetermined threshold Sth. The threshold Sth is determined by taking into consideration the magnitude of the reception error of the quasi-synchronized detection signal R (t) or the like. If the power value (L/Fs).sup.2 is larger than the threshold Sth, the UW detector 105 produces a UW detection signal S108 indicating that a UW is detected from the quasi-synchronized detection signal S100. The UW detection signal S108 is used as a frame synchronization signal in synchronization demodulation of the quasi-synchronized detection signal S100, or the like.
A UW phase arithmetic unit 106 calculates a phase term .theta. from the Re and Im contained in the cross correlation signal S107 to generate frequency offset information S109. Here, since the UW phase arithmetic unit 106 calculates the phase term .theta. of a cross correlation function D (t) as tan.sup.-1 (Im/Re), the determinable upper limit of phase term .theta. is .+-..pi.. Therefore, the measurement range of the frequency offset F0 becomes .+-.Fs/2 or less for Ts=1 (N=1) symbol time.
FIG. 3 shows a measurement rage of the frequency offset F.sub.0 for N symbols, amount of delay for the quasi-synchronized detection signal R (t) and the unique word U (t) in the UW differential detector circuit 100. That is, the measurement range of the frequency offset F.sub.0 is inversely proportional to the number of delay symbols N. On the other hand, resolution of the frequency offset F.sub.0 (frequency resolution) would be proportional to the amount of delay N. The frequency offset information S109 is used as correction information for carrier frequency offset in the synchronous demodulation of the quasi-synchronized detection signal S100, or the like.
The above-mentioned conventional unique word differential detection system has features not only being capable of detecting a unique word even if there is a carrier frequency offset, but also being fast in detecting the unique word.
However, the conventional unique word differential detection system has the first problem that, as shown in FIG. 3, the measurement range of carrier frequency offset is in a relationship of tradeoff with the resolution, so that both cannot be simultaneously enhanced.
The second problem lies in that the unique word differential detection system does not have a measure for protecting false detection of the unique word.
In addition, the third problem lies in that, when the carrier frequency offset has a high magnitude, the unique word differential detection system cannot prevent a unique word detection capability from being deteriorated due to noise and adjacent channel interference (ACI), and noise containing ACI from being increased in a signal supplied to a demodulator.
Furthermore, the forth problem lies in that in estimating timing of the unique word by the unique word differential detection system, signal processing speed is reduced when estimation accuracy is increased.